FUNDAMENTAL HYPOTHESES . 37
however, it is necessary for the student to have some concrete
mental picture of this structure there is no reason why he
should not employ an image of the nervous system, provided
that he does not rashly assume that the material nervous
structure is the mental structure.
The main source of trouble is that, however^ we seek to
describe our hypothesis, we cannot avoid the use of metaphors;
the words 'background* and 'structure' bring in their train
many ideas, some relevant, others irrelevant. Metaphors are
dangerous things, and in psychology they are particularly apt
to be misleading, since they are drawn from the material
world. Yet they cannot be dispensed with, and, if we realize
that they under-dcscribe and over-describe at the same time,
and if we arc not afraid to mix our metaphors when necessary,
no great harm need be done. Bearing these considerations
in mind, and remembering that it is permissible to modify a
hypothesis so that it may better describe our facts, we may
proceed to a preliminary examination of our hypothesis of
'background* or 'structure.' The main point to be grasped
now is that the 'structure,* whatever its nature, is part of a
living organism, and that one of its essential attributes is
activity; it is dynamic, not static. If it is difficult to think of
a structure as active, not only rearranging its own constituent
parts, but actually determining its own functioning, then we
must realize that it is here that our inadequate metaphor
breaks down. My active mental structure lies behind my
experience and behaviour, determining what form they will
take. It is my mental structure, for example, that determines
me to experience fear in a dangerous situation, and to run
away, or, at least, to experience an impulse to run away.
This hypothesis of mental structure has been accepted in
recent years by many authorities in the psychological world,
but different words have been used to describe it. Of such
words 'disposition* is perhaps the most important. Stout, for
example, points out that the psychologist is
compelled at every step to recognize the existence of what arc
called psychical or mental dispositions, inherited and acquired.
Our actual experience at any moment is determined by conditions